disk/lvm: Don't blast past the end of the circular metadata buffer

This catches at least some OOB reads, and it's possible I suppose that
if 2 * mda_size is less than GRUB_LVM_MDA_HEADER_SIZE it might catch some
OOB writes too (although that hasn't showed up as a crash in fuzzing yet).

It's a bit ugly and I'd appreciate better suggestions.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/disk/lvm.c

@@ -251,6 +251,16 @@ grub_lvm_detect (grub_disk_t disk,
   if (grub_le_to_cpu64 (rlocn->offset) + grub_le_to_cpu64 (rlocn->size) >
       grub_le_to_cpu64 (mdah->size))
     {
+      if (2 * mda_size < GRUB_LVM_MDA_HEADER_SIZE ||
+          (grub_le_to_cpu64 (rlocn->offset) + grub_le_to_cpu64 (rlocn->size) -
+	   grub_le_to_cpu64 (mdah->size) > mda_size - GRUB_LVM_MDA_HEADER_SIZE))
+	{
+#ifdef GRUB_UTIL
+	  grub_util_info ("cannot copy metadata wrap in circular buffer");
+#endif
+	  goto fail2;
+	}
+
       /* Metadata is circular. Copy the wrap in place. */
       grub_memcpy (metadatabuf + mda_size,
 		   metadatabuf + GRUB_LVM_MDA_HEADER_SIZE,